LONDON — One of the rare occasions on which I’ve encountered Dominic Cummings, who may be the most important man in Britain right now, was at a private political dinner in 2016, just after the plan to hold a referendum on Britain’s membership in the European Union had been announced. Mr. Cummings, who a few years earlier had been a political adviser at the education ministry, had been invited to outline the argument that the Brexiteers would be making; a counterpart from the fledgling Remain campaign was there to present his.

The Remainer, a powerful, confident and well-connected man, went first. He presented a baffling, limited, sterile case: Britain had a thriving and critical car industry, which would see its profit margins wiped out by tariffs if Brexit went ahead.

I detested Brexit and all it stood for, but I was captivated. Mr. Cummings was making it sound like the noble path. I came home anxious and uneasy. Remainers were way ahead in the polls, but would they come up with something effective to combat the deep emotions that Mr. Cummings’s campaign was tapping into?

They never did. Mr. Cummings went on to drive Brexit, pushing it to a narrow victory against huge internal opposition, by focusing aggressively on what worked. He outwitted Britain’s establishment by combining a brilliantly simple slogan — “Take back control” — with shameless lies about the European Union, the National Health Service and the danger that Turks could soon emigrate to Britain en masse, all backed up by a huge and hidden microtargeted social media campaign. Every element was designed to have a powerful, visceral appeal.

Mr. Cummings proved that stories and lies, allied to strategic cunning, conviction, secrecy, ruthlessness and upending convention, could be much more appealing than reason and fact. Years of studying and writing obsessively about the art of strategy, the failings of most institutions and the success of revolutionary thinkers like Otto von Bismarck had paid off.

Now this single-minded insurgent is the most powerful individual in the British government, vaulted into Downing Street as Prime Minister Boris Johnson’s chief strategic adviser. His job is to deliver Brexit and win Mr. Johnson five years more in office, making up for the prime minister’s deficiencies as a lazy, inattentive bumbler. Mr. Cummings is deploying all the techniques that have worked for him before: disruption, deception, intimidation and an implacable willingness to alienate people.

Those tactics have gone off in British politics this week like a series of grenades. No prime minister has had a more calamitous start. These are the opening battles of a seismic political war. Within five days, Mr. Johnson has lost control of Parliament, lost his party’s majority, lost his ability to leave the European Union at the end of October without a deal, as he has pledged, and lost the loyalty of many moderates in his Conservative Party by brutally expelling the 21 principled, thoughtful, experienced and respected members of Parliament who had opposed his plans to catapult Britain out of Europe without a deal.

Mr. Cummings’s decisions have left Mr. Johnson dangling at the mercy of the opposition parties. He can’t pass any laws, and the opposition now has the power to make the crucial political decision: the date of the next election.

These convulsions have caused uproar in the Conservative Party’s ranks. The former prime minister John Major is demanding Mr. Cummings’s sacking, saying he’s a “political anarchist” who must be ousted before he poisons the government “beyond repair.” Incensed and appalled members of Parliament are watching their party morph before their eyes into a hard-line vehicle for the most intransigent, right-wing Brexiteers, in which centrists and Remainers are welcome so long as they shut up and do as they are told. A senior party figure told me, with classic English understatement, that this is “a ghastly mess.” “I’m not sure that Boris read the small print of the Cummings plan — this is the ultimate proof that one is a charlatan and the other a psychopath,” he said.

This looks like a catastrophe for Mr. Johnson. But the story may not be so simple. Mr. Cummings deliberately framed and precipitated the confrontation with Parliament, intending to lose the vote so that Mr. Johnson could instantly call an election as the people’s champion, the deliverer of Brexit, the supporter of no deal. He didn’t expect so many Tories to revolt or the opposition to derail his timetable for an election, but Mr. Cummings, who considers himself a master strategist, sees these as little more than skirmishes before the real fight.

He is unmoved by the indignation, the denunciations, the many enemies his contemptuous, bullying behavior has made. He calculates that pro-Brexit voters far from the financial and political elite in London will see the prime minister’s ruthlessness as proof he’s on their side against obstructive metropolitan Remainers. He believes his strategy, to win an election this autumn by making nakedly populist pledges and stealing votes from the insurgent, radical Brexit Party’s votes, can still work.

Yet one of the weakest links in this plan is the character of the leader, the man whom Mr. Cummings is using as the vehicle for his destructive strategy. If Mr. Cummings’s plan is to divide the Conservative Party from the inside out, Boris Johnson is his co-conspirator as his careless, indolent host. But the person who has been left utterly shellshocked by events is Mr. Johnson himself.

Where Mr. Cummings is a steely ideologue, Mr. Johnson doesn’t enjoy conflict; he wants power accompanied by endless applause. He never expected to have to expel senior members of his party; he expected them to be won over by his charm. He was humiliated by the scorn heaped on him in Parliament on Tuesday, where he was nervous and out of his depth. On Thursday came the most damning condemnation: His brother Jo Johnson, a moderate, resigned from the government, accusing Boris of damaging the national interest.

This was such a blow that sources close to the prime minister tell me that he cried when he heard the news. The speech he gave a couple of hours later, originally planned as the kickoff of an election campaign, was a delayed, confused and extraordinarily rambling mess. The qualities that Tory members were hoping for when they gave him the leadership — charisma, confidence, wit and sunny optimism — have been dangerously absent.

Mr. Cummings now faces a fight to keep Mr. Johnson on form and on track in the face of tremendous blowback from an outraged party. But he’s still the master; he calculates that Mr. Johnson can’t afford to lose him now that he has cut so many of his old allies out. Whether Mr. Johnson is heading for either triumph or disaster isn’t up to Mr. Johnson. His course is being set by Dominic Cummings.

A version of this article appears in print on , Section SR, Page 3 of the New York edition with the headline: The ‘Political Anarchist’ Behind Britain’s Chaos. Order Reprints | Today’s Paper | Subscribe